1. Field of the Invention
The present invention relates to a spot-size converter that is used in optical connection between a silicon wire waveguide and an external optical system such as a laser diode or an optical fiber.
2. Description of the Related Art
In recent years, technologies that use silicon as an optical waveguide material have been gathering attention. Among these technologies, there is particularly known an optical waveguide called a silicon wire waveguide that uses silicon as its core and uses silicon dioxide, whose refractive index is extremely smaller than that of silicon, as its clad.
A silicon wire waveguide can strongly trap light in its core because the difference in refractive index between the core and the clad is extremely large. As a result, by using a silicon wire waveguide, manufacture of a compact curved waveguide whose bend radius is made as small as about 1 μm, for example, can be realized, and an optical device with dimensions of an extremely minute submicron order can be created.
For that reason, silicon wire waveguides have been attracting attention as a technology having the potential to be able to incorporate a silicon electronic device and a photonic device on the same chip.
Incidentally, in order to perform connection between a silicon wire waveguide and an external photonic device such as a laser diode (hereinafter also called an LD) or an optical fiber, for example, a spot-size converter is placed between these. This is because the diameter of the core of the silicon wire waveguide is extremely small as compared to common external photonic devices such as an LD or an optical fiber. For that reason, it is necessary to reduce the spot size of light that is inputted to the silicon wire waveguide from the LD or to magnify the spot size of light that is outputted to the optical fiber from the silicon wire waveguide by using a spot-size converter.
As such a spot-size converter, various proposals have conventionally been made. For example, spot-size converters where the width of the optical waveguide is made smaller—that is, is made narrower—in a tapered manner toward an input/output end face are well known (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 2002-162528, JP-A No. 2000-235128, U.S. Pat. No. 6,684,011, and JP-A No. 2003-2076834). Further, spot-size converters where the thickness of the optical waveguide is made smaller—that is, is made thinner—in a tapered manner toward an input/output end face are well known (e.g., see JP-A No. 9-15435 or JP-A No. 2005-326876).
Here, in these spot-size converters, when the input/output end face of the spot-size converter and the end portion of the core on this input/output end face side are spaced apart from each other, there is the fear that the light will spread between the input/output end face and the end portion of the core and that the light will leak out to the substrate on which the core and the clad are formed. As a result, coupling efficiency can worsen when the light is inputted from the LD to the silicon wire waveguide, for example. In order to eliminate this problem, conventionally, a structure that enhances the light trapping effect by including the optical waveguide in a second core—that is, a double core structure—is well known (e.g., see JP-A No. 7-63935).
However, it is difficult to manufacture a spot-size converter with such a double core structure, and the costs of manufacturing a spot-size converter with such a double core structure increase.